This application claims the priority of German Patent Application Serial No. 101 23 964.5, filed May 17, 2001, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates to a switchable tappet for directly transmitting a cam lift onto a tappet push rod, especially in a valve train of an internal combustion engine.
German Pat. No. 198 44 202 describes a tappet of a type involved here, including a housing formed with a recess for receiving an inner element for movement in longitudinal direction and abutment by a push rod. At a side distal to the push rod, the housing has a ramp surface for contact by a cam. The inner element is extended beyond the recess of the housing and is acted upon by a spring assembly comprised of two compression springs and received in the housing. The spring assembly rests on one end against a housing bottom and on the other end against an underside of the inner element. This conventional tappet suffers shortcomings because the arrangement of the spring assembly results in an unnecessarily long housing length, as a result of the required spring deflection plus the block size of the spring assembly. Thus, the tappet is overall of long dimension between its abutment against the inner element for the push rod and the cam ramp surface, so that the total mass is increased and the overall construction is complicated. Moreover, manufacturing costs are increased, as a number of outer surface areas are not smooth and of long size, and require cost-intensive fine machining operations, such as grinding.
It would therefore be desirable and advantageous to provide an improved switchable tappet to obviate prior art shortcomings.
According to one aspect of the present invention, a switchable tappet for directly transmitting a cam lift onto a tappet push rod, in particular in a valve train of an internal combustion engine, includes a housing formed with a recess and having a push rod distal end which is formed with a ramp surface for contact by a cam, an inner element disposed in the recess for movement into at least one position in which the inner element is coupled to the housing, wherein the inner element has a push rod proximal end formed with an abutment for the push rod, an attachment secured to the push rod proximal end of the inner element to extend the inner element beyond the recess of the housing, wherein the attachment has a housing-distal end formed with a shoulder, and a helical compression spring, extending between the shoulder of the attachment and a rim or rim-proximal region of the housing, for urging the inner element in a direction of the push rod, and conically expanding, at least partly, in the direction of the push rod and wrapped around the attachment of complementary conical configuration.
The present invention resolves prior art problems by shifting the arrangement of the compression spring to the outside so that the housing and thus the entire tappet can be constructed significantly more compact compared to conventional tappets. As a consequence, the overall mass of the tappet can easily be reduced. The conical configuration of the compression spring with its attachment contributes to a further shortening of the overall tappet length, when compared to conventional helical cylindrical compression springs. The conical configuration also ensures a sufficient mobility of the push rod upon the abutment during operation of the valve drive. Of course, the free mobility of the push rod may also be established by manufacturing only the inner surface area of the attachment in a conical way.
Despite the conical configuration of the attachment, it may also be possible to make the compression spring cylindrical or partly cylindrical for certain applications. It is also conceivable to provide a conical spring assembly with compressions springs nested within one another.
Measured between the abutment for the push rod and the ramp surface for the cam, the tappet should have a length which at a maximum is about ⅔ of a length of conventional tappets for valve drives in internal combustion engines of this type.
According to another feature of the present invention, the attachment may be a separate structural element and made of thinwalled lightweight material to keep the overall weight to a minimum. Examples of lightweight material for making the attachment include sheet metal or plastic. Suitably, the shoulder of the attachment is formed in one piece with the attachment and configured as ring collar extending from the attachment. Of course, the shoulder may also be a separate part that is connected to the attachment.
According to another feature of the present invention, the attachment may be connected to the outer or inner surface area of the inner element by a clip connection or snap connection. Such a connection is easy to realize. Other types of connection at the disposal of the skilled artisan may be suitable as well. Of course, it is also conceivable to provide the attachment in single piece configuration with the inner element.
A simple securement of the attachment can be realized by providing the inner element with a necking in an axial area of the rim of the housing. Suitably, the attachment is secured to the outer surface area of the inner element in the area of the necking. Of course, it is also conceivable to secure the attachment to the inner surface area of the inner element.
According to another feature of the present invention, the inner element has a ramp surface proximal side which is provided with a cylindrical impression whereby the ramp surface for the cam may be implemented as a rolling bearing mounted roller. In this way, the overall size of the tappet is further reduced, because part of the end face of the inner element moves over the roller as ramp surface in off-mode of the tappet.
According to another feature of the present invention, there is provided a hydraulic clearance compensation element, which is disposed in the inner element. As a consequence, additional adjustment measures for the valve clearance in the valve train, also of mechanical type, can be omitted. Supply of the hydraulic clearance compensation element may be implemented selectively via the push rod or a guide member enclosing the housing. The same is true for the coupling means, not described in more detail here.
According to another feature of the present invention, the housing may have a thinwalled structure and exhibit at least over a major part thereof a continuously open, tubular configuration.